The present invention relates to a real time histogram modification system for use in image processing, particularly image contrast enhancement.
In the prior art it is known to produce image contrast enhancement off-line using optical techniques and digital computers. In the case of using computers, input grey level information is operated on using various transformations or operators such as histogram equalization and histogram hyperbolization prior to the transformed information being supplied to a signal output. A histogram equalization technique is disclosed by R. Hummel, Image Enhancement by Histogram Transformation, Computer Graphics and Image Processing 6, 184-195 (1977) and a histogram hyperbolization technique is disclosed by Werner Frei, Image Enhancement by Histogram Hyperbolization, Computer Graphics and Image Processing 6, 286-294 (1977).
Two techniques for image enhancement in real time are disclosed by David J. Ketcham, Real Time Image Enhancement Techniques /SPIE/OSA--Vol. 74 (1976). Image Processing and these techniques are described in greater detail in U.S. Pat. Nos. 3,983,320 and 3,996,421. One of these techniques concerns local area histogram equalization (LAHE) and the other concerns local area enhancement by adjusting video gain and brightness. The LAHE technique is suited for use in those situations where it is desired to eliminate contrast attenuation in image areas that represent a statistically small but important portion of a grey scale histogram. In this technique a histogram of the intensity values of an area of say 32.times.32 pixels is computed and a recoding strategy is determined on the basis of histogram equalization, this strategy then being applied to the central 4.times.4 pixels of the area under consideration. The area, which may be termed a window or keyhole, is moved by 4 pixels along the same line and the process is repeated with the new image enhanced 4.times.4 area being adjacent the first area. This process continues 4 pixels at a time along a line until no further horizontal movement is possible. The system then drops down 4 pixels and continues the process step by step horizontally. Ultimately, apart from a margin comprising a peripheral band of 14 pixels deep, the remainder of the picture has undergone local area histogram equalization. Although the technique is in itself satisfactory, the system is structurally complex and requires a great deal of circuitry, especially memories. In the gain/brightness technique (U.S. Pat. No. 3,996,421) a smaller sliding window of 8.times.8 pixels is taken and the intensity value of only one pixel is adjusted.